Old St. Hilary's Landmark is a former mission church place of worship and now a historic landmark owned and maintained by the Belvedere-Tiburon Landmarks Society, Tiburon Peninsula, Marin County, California, United States. The landmark is situated on the flank of the Tiburon Hills, an area characterized by serpentine soils and hence has a distinct plant community. The area surrounding the church is a Marin County protected area, the Old Saint Hilary's Open Space Preserve.